The Official Publication of the InterNational Electrical Testing Association
Winter 2005-2006
Tech Brief
lating of Contact Surfaces
P
in Switchgear and Circuit Breakers
T
he various ANSI standards covering metal-enclosed switchgear and
circuit breakers used in this switchgear prescribe temperature rise
limits for various parts of the equipment. Included are limitations
for buses with plated and unplated joints in switchgear and for plated
and unplated contacts and connections in circuit breakers. In all cases the
temperature rise allowed is considerably higher for plated connections
than it is for unplated connections. Typically, the limit for unplated copper
connections is 30ºC rise, while the limit for plated connections is 65ºC rise.
The higher temperature rise is allowed for plated connections because
plated copper does not oxidize nearly as rapidly as bare copper. Copper
oxide is not a good conductor, and once the oxide forms, the resistance
and the temperature rise of the conductor usually increase rapidly. Since
limiting the temperature rise to 30ºC would require manufacturers to
double the amount of copper used, joints are almost universally plated.
The two materials commonly used for plating are silver and tin. The
standard for high voltage circuit breakers speaks of “silver, silver alloy, or
equivalent” surfaces with “equivalent” being undefined. This standard
was last revised in 1979. The low-voltage and medium-voltage switchgear standards, revised in 1987, speak of “silver surfaced, tin surfaced,
or equivalent” connections.
NETA WORLD Winter 2005-2006
by Baldwin Bridger
Formerly of Powell
Electrical Manufacturing Co
Which material is better, silver
or tin? At Powell, we generally
use silver, particularly for sliding
contacts. Silver plating is harder
than tin plating and withstands
the stress of a moving joint, such
as a hinge point or a primary disconnect, better than tin plating.
However, tin plating is superior
in certain industrial atmospheres,
such as those containing hydrogen
sulfide. On request, Powell will
provide tin plating on the connections of the bus bars in equipment.
For various technical and manu1
facturing reasons, it is not practical to substitute tin for
silver on surfaces within circuit breakers or on circuit
breaker primary disconnects. If the atmosphere attacks
silver surfaces, they should be coated with contact
lubricating grease to prevent corrosion problems.
Editor’s Note: Although the standards referred to have been
revised in years later than indicated, the temperature ratings
have not changed.
Baldwin Bridger, PE, is retired Technical Director of Powell
Electrical Manufacturing Co., Houston, Texas. He has worked as
an engineer and engineering manager in the design of low- and
medium-voltage switchgear since 1950, first at GE and since 1973
at Powell. He is a Fellow of IEEE and a past president of the IEEE
Industry Applications Society
Article available for reprint with permission from NETA. Please inquire at neta@netaworld.org.
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NETA WORLD Winter 2005-2006